FIG. 1 shows a prior art instrumentation amplifier that includes three op-amps. Instrumentation amplifier comprises two op amps, A1 and A2, which operate as “gain cells” and are suitably connected together so that a differential input signal is applied to the non-inverting inputs of the two op amps, while the outputs of the two op amps are applied as a differential input to a third op amp, A3, that is connected as a difference amplifier. Instrumentation amplifier shown in FIG. 1 required three gain blocks and precise matching of resistors R4 through R7.
One approach for an instrumentation amplifier which does not require three gain blocks is disclosed by Bowers in “An Ultra-Low-Noise Monolithic Microphone Preamplifier” presented at AES 83rd Convention, October 1987 (paper No 2495) and by Nelson in “Wideband Feedback Amplifier”, U.S. Pat. No. 4,628,279. This topology is presented in FIG. 2. Input transistors Q1 and Q2 form a differential pair, loaded by R1 and R2. Emitters of the input devices are connected together with resistor Ra. Any imbalance in collector currents is sensed by amplifier A1 and corrected through feedback resistor R5 to the emitter of Q2. R3 provides dc balance to the system and is equal in value to R5. Both input devices have finite resistance signal path from their emitter to common terminal/output (R3 and R5). A common mode input signal changes the current through both input transistors Q1 and Q2. This change may cause unnecessary degradation of the common mode rejection ratio and additional distortion of the amplified signal.
Elevated input capacitance is another drawback of this circuit. Miller effect accounts for the increase in the input capacitance of the input devices due to amplification of the capacitance between the base and collector terminals. Junction capacitance depends of the applied voltage. Input capacitance variation with the input signal introduces additional distortion to the amplifier.